1. Field of the Invention
The present invention relates to an optical apparatus and an image pickup apparatus, and more particularly, though not exclusively, to an image pickup apparatus capable of image pickup in at least two wavelength bands.
2. Description of the Related Art
A color image includes color information in addition to information of shape and brightness and is therefore useful in distinguishing a photographed object in a color image compared to a monochrome image. Therefore, normally, security cameras photograph color images. However, image sensors, such as a CCD, widely used in security cameras are sensitive to not only light in the visible wavelength range (between approximately 400 nm and 700 nm) but also light in the near-infrared wavelength range (more than approximately 700 nm). Therefore, if a color image is photographed using the information of all ranges to which the CCD is sensitive, the colors of the photographed color image is out of balance. Thus, the colors of the subject cannot be accurately reproduced. To solve this problem, when a color image is photographed, a near infrared light cut filter (hereinafter referred to as “IR cut filter”) for cutting near infrared light is disposed in the optical path of the photographing light.
However, in the case of photographing a very dark subject, for example, at night outdoor or in a room with no illumination, the amount of incident light is insufficient. Therefore, the amplification factor of the image signal output from the image sensor is increased. However, large amplification of the color image signal deteriorates the signal-to-noise (S/N) ratio of the color signal and results in a noisy image. Thus, the outline or the shape of the subject becomes unclear. In addition, since color information decreases at low illuminance, the value of the color image decreases.
Therefore, in the case of photographing at low illuminance, conventional systems typically abandon obtaining color information deteriorating the S/N ratio, and image processing is performed on the basis of a monochrome image comprising luminance signals only. That is to say, the IR cut filter is removed from the optical path, and instead a dummy glass for correcting the optical path length is inserted. Since near infrared light can be added to the luminance information, it is possible to photograph the subject more clearly and brightly even at low illuminance. Such a photographing mode is called “night mode” and is discussed in Japanese Patent Laid-Open No. 2001-45512.
Japanese Patent Laid-Open No. 2001-45512 also discusses a technique called “automatic night mode” of automatically switching between the color and monochrome photographing modes. In addition, Japanese Patent Laid-Open No. 2000-224469 discusses a technique of automatically switching to the night mode when the illuminance of the subject decreases using an illuminance sensor for detecting the visible light illuminance of the subject provided separately from the image pickup optical system.
However, the above conventional image pickup apparatus, which can have the automatic night mode, can fail to perform appropriate automatic night mode operation. The first cause thereof is misdetection of the illuminance. The reason of the misdetection is, for example, that the illuminance detection range (angle of view) is larger than the imaging range (angle of view) of the image pickup optical system, or that the illuminance detection range does not exist within the imaging range of the image pickup optical system. No appropriate relationship between the illuminance detection range of the illuminance detection device and the imaging range of the image pickup optical system has been discussed.
The second cause is that the relationship between the imaging range (angle of view) changing with zoom operation and the timing when the IR cut filter is inserted or removed is inappropriate. Therefore, in the case where the imaging range and the illuminance detection range of the illuminance detection device change relative to each other, the apparatuses can fail to perform appropriate automatic night mode operation.